Optical storage system divider based draw verification with high frequency writing strategy pattern

ABSTRACT

An optical storage system includes an optical head configured to split a light beam into a higher power main beam and at least one lower power side beam. The optical storage system also includes a controller configured to alter an optical medium, via modulation of the higher power main beam according to a writing strategy waveform that defines at least n pulses for every n bits of data to be written to the medium, while processing a first signal resulting from the at least one lower power side beam being reflected from the medium and a second signal indicative of the writing strategy waveform to remove noise from the first signal caused by the higher power main beam to generate output indicative of the data directly after writing.

INCORPORATION BY REFERENCE; DISCLAIMER

The following application is hereby incorporated by reference:application Ser. No. 15/355,891 filed on Nov. 18, 2016. The Applicanthereby rescinds any disclaimer of claim scope in the parent applicationor the prosecution history thereof and advises the USPTO that the claimsin this application may be broader than any claim in the parentapplication.

TECHNICAL FIELD

This disclosure relates to techniques for real-time verification ofwritten data in an optical storage system.

BACKGROUND

Optical recording devices such as optical disk and optical tape drivescommonly use an Optical Pickup Unit (OPU) or read/write head to writeand retrieve data from associated optical media. Conventional OPUs mayutilize different wavelength semiconductor laser diodes with complexbeam path optics and electromechanical elements to focus and track theoptical beam within one or more preformatted tracks on the medium towrite or store the data and subsequently read the data. Data written tothe medium with a laser at higher power may be verified in a separateoperation or process after writing using a lower laser power, or may beverified during the write operation by another laser or laser beam. Theability to read and verify the data during the write operation may bereferred to as Direct Read After Write (DRAW).

Current OPUs may use a diffraction grating or similar optics in thelaser path to generate three beams from a single laser element includinga higher power beam used for reading/writing data and for focusing, andtwo lower power satellite beams used for tracking. The three beams arefocused to three corresponding spots on the surface of the opticalstorage medium used by the various optical and electromechanicalelements of the OPU. In general, the higher power spot is positioned inthe center or middle between the two satellite spots. In addition toreading/writing data and focusing, the center spot may also be used forone particular type of tracking operation in some applications. Thelower power satellite spots generated from the lower power side beamsare typically used for another type of tracking operation for specifictypes of media.

SUMMARY

Optical storage systems and methods of performing direct read afterwrite for the same utilize circuitry and/or controllers configured toprocess data read directly after writing to remove noise introduced bythe writing. Because the writing process involves high-frequency writingstrategy pulses in the laser's optical power for creating the crystalphase change on the optical recording layer of the media, the directread laser power signal from the laser light sensor during the writecontains modulation of the written data and the high-frequency writingpulses. Although division of the read signal by the writing strategysignal, for example, can cancel out the noise to recover and verify thewritten data, the number of pulses used to write a given number of bitshas been found to impact the quality of the verification mentionedabove.

In one embodiment, an optical storage system includes an optical headand controller arrangement that modulates a higher power main beamaccording to a writing strategy waveform that defines at least one pulsefor each bit to be written to an optical medium, reads, directly afterwriting, feedback from the medium containing written data and noiseresulting from the higher power main beam, removes the noise from thefeedback by dividing the feedback with data indicative of the writingstrategy waveform, and generates output indicative of the written data.

In another embodiment, a method for performing direct read after writeon an optical medium includes splitting a light beam into a higher powermain beam and at least one lower power side beam, and modulating thehigher power main beam according to a writing strategy waveform thatdefines at least n pulses for every n bits of data to be written to theoptical medium. The method also includes, while performing themodulating, processing a first signal resulting from at least one of thelower power side beams being reflected from the medium and a secondsignal resulting from scatter of the higher power main beam to removenoise from the first signal caused by the higher power main beam, andgenerating output resulting from the processing indicative of the data.

In yet another embodiment, an optical storage system includes an opticalhead that splits a light beam into a higher power main beam and at leastone lower power side beam, and a controller. The controller alters anoptical medium, via modulation of the higher power main beam accordingto a writing strategy waveform that defines at least n pulses for everyn bits of data to be written to the medium, while processing a firstsignal resulting from the at least one lower power side beam beingreflected from the medium and a second signal indicative of the writingstrategy waveform to remove noise from the first signal caused by thehigher power main beam to generate output indicative of the datadirectly after writing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams illustrating operation of an exampleoptical data storage system or method with direct read after write(DRAW) capability.

FIG. 2 is a block diagram illustrating operation of the optical pickupunit (OPU) of FIGS. 1A and 1B having a coherent light beam split ordivided into a center beam and two satellite or side beams to provideDRAW capability.

FIGS. 3A through 3C are diagrams illustrating components associated withRF and FM signal wave forms, and the result of their division.

FIG. 4 is another block diagram illustrating operation of the exampleoptical data storage system of FIGS. 1A and 1B.

FIG. 5 is a block diagram illustrating an example DRAW demodulationcircuit.

FIG. 6 is a plot of an example 5T space and 4T mark laser pulse writingstrategy.

FIG. 7 is a plot of the Welch power spectral density associated with aread-back signal, and an RF signal distorted by a writing strategysignal that uses the coding techniques proposed herein.

FIG. 8 is a plot comparing, for the same data, a read signal generatedduring a read operation (bottom waveform) and a DRAW division outputsignal generated by the DRAW demodulation circuit of FIG. 5 during awrite operation (top waveform).

FIG. 9 is a plot comparing signal-to-noise ratio (SNR) performance ofthe proposed and conventional writing strategies over a number of lowpass filter bandwidths.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are described herein.However, the disclosed embodiments are merely exemplary and otherembodiments may take various and alternative forms that are notexplicitly illustrated or described. The Figures are not necessarily toscale; some features may be exaggerated or minimized to show details ofparticular components. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a representative basis for teaching one of ordinary skill inthe art to variously employ the present invention. As those of ordinaryskill in the art will understand, various features illustrated anddescribed with reference to any one of the Figures may be combined withfeatures illustrated in one or more other Figures to produce embodimentsthat are not explicitly illustrated or described. The combinations offeatures illustrated provide representative embodiments for typicalapplications. However, various combinations and modifications of thefeatures consistent with the teachings of this disclosure may be desiredfor particular applications or implementations.

The processes, methods, logic, or strategies disclosed may bedeliverable to and/or implemented by a processing device, controller, orcomputer, which may include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, logic, or strategies may be stored as data and instructionsexecutable by a controller or computer in many forms including, but notlimited to, information permanently stored on various types of articlesof manufacture that may include persistent non-writable storage mediasuch as ROM devices, as well as information alterably stored onwriteable storage media such as floppy disks, magnetic tapes, CDs, RAMdevices, and other magnetic and optical media. The processes, methods,logic, or strategies may also be implemented in a software executableobject. Alternatively, they may be embodied in whole or in part usingsuitable hardware components, such as Application Specific IntegratedCircuits (ASICs), Field-Programmable Gate Arrays (FPGAs), statemachines, controllers or other hardware components or devices, or acombination of hardware, software and firmware components.

Referring now to FIGS. 1A and 1B, block diagrams illustrating operationof an example optical data storage system or method with direct readafter write (DRAW) capability are shown. FIG. 1A is a side view diagramand FIG. 1B is a top or plan view diagram. In this embodiment, opticaldata storage system 10 is an optical tape drive 12 that receives anoptical data storage medium 14, which is an optical tape 16. Whileillustrated and described with reference to an optical tape drive, thoseof ordinary skill in the art will recognize that the teachings of thepresent disclosure may also be applied to various other types of opticaldata storage devices that may use various types of write-once orre-writable optical media, such as optical discs. The optical tape 16 isa ½ inch (12.7 mm) wide tape having a plurality of tracks 36 generallyextending across the width of the tape and may vary in length dependingon the desired storage capacity and performance characteristics asillustrated and described in greater detail herein. Other tapeconfigurations and dimensions, however, are also possible. The opticaltape 16 may be wound on an associated spool 17 contained within aprotective case or cartridge 18 that is manually or automatically loadedor mounted in the optical tape drive 12. Transport mechanism 24 movesthe optical tape 16 through a carriage and past at least one opticalpickup unit (OPU) or optical head 20 to a take-up spool 22 thattypically remains within the tape drive 12. The OPU 20 writes data to,and reads data from, the optical tape 16 as the transport mechanism 24moves the optical tape 16 between the cartridge 18 and take-up spool 22in response to at least one controller and associated electronics 26. Asexplained in greater detail below, data may be read/written to theoptical tape 16 in one or more of the tracks 36 in a serpentine fashionas the tape travels in either direction past the OPU 20, i.e., eitherfrom the cartridge 18 to the take-up spool 22, or from the take-up spool22 to the cartridge 18.

The optical head 20 may include associated optics and relatedelectromechanical servo controlled devices, represented generally byreference numeral 30, that split or divide a light beam, such as a laserbeam, into two or more beams that are focused to corresponding spots onthe storage medium 16 for reading/writing data as illustrated anddescribed in greater detail with reference to FIG. 2. Various servomechanisms (not specifically illustrated) may be used to position/alignthe beams with a selected one of the tracks 36 on the optical tape 16.

FIG. 2 is a block diagram illustrating operation of the optical pickupunit (OPU) 20 having a coherent light beam split or divided into acenter beam 40 and two satellite or side beams 44, 48 to provide DRAWcapability. The beams 40, 44, 48 may be generated by a single or commoncoherent light source, such as a laser diode, for example. The sourcebeam travels through associated optics, that may include a diffractiongrating, for example, to divide or split the source beam into the centerbeam 40, first side beam 44, and second side beam 48 and to focus thebeams to corresponding spots 50, 54, and 58, respectively, on thesurface of the optical tape 16 within a selected one of the tracks 36.The three optical spots 50, 54, and 58 are manipulated by variousoptical and electrometrical elements of the OPU 20 to write and retrievedata from the optical tape 16.

The optical elements used to split the source beam and focus theresulting beams to the spots 50, 54, and 58 may be designed to providehigher power to the center beam 40 and center spot 50 with lower powerto the side beams 44, 48 and associated spots 54, 58. For example, thecenter beam 40 may contain about 90% of the source beam power with theside beams 44, 48 dividing the remaining 10% of the source beam power.The center beam 40 is modulated by the OPU 20 to generate write marks 60during writing of data to the optical tape 16, which may require aboutten times more average power than to read previously stored data (about7 mW to write data and about 0.3 mW to read data, for example). As such,if the source beam is modulated and produces sufficient power forwriting data using the center beam/spot 40/50, the side beams 44, 48will be modulated in a like manner but will contain insufficient powerto alter the tape 16.

In this embodiment, the spots 50, 54, and 58 are mechanically aligned inthe OPU manufacturing process to correspond to the axes of the datatracks 36. In addition, the satellite spots 54, 58 are generallysymmetrically positioned relative to the center spot 50 so that transitdistance (d) of the tape 16 between the center spot 50 and either of thesatellite/side spots 54, 58 is substantially the same. Other embodimentsmay include a distance (d) of between about 10-20 μm, although otherdistances are also contemplated.

Certain conventional optical storage devices use the center spot 50 fromthe higher power emitting beam 40 for reading, writing, and focusing inaddition to one type of tracking operation. The satellite spots 54, 58formed by the lower power side beams 44, 48 are used for another type oftracking for specific types of media. In these applications, the sidespots 54, 58 may not be aligned with one another, or with the centerspot 50 along a single one of the tracks 36.

As previously described, the source laser beam is operated at a higherpower (relative to operation during a data read/retrieval) and modulatedto write the data marks 60 on a selected one of the tracks 36 on theoptical tape medium 16. However, only the center beam 40 emits enoughpower to the optical tape 16 to actually alter the structure of theoptically active layer. The satellite beams 44, 48, having much lowerpower as determined by the diffraction grating power distribution, donot alter the tape 16. They, however, have enough power after beingreflected from the optical tape 16 to detect the data marks 60.Therefore depending on the direction of travel of the optical tape 16,the reflection from one or both of the associated satellite spots 54, 58can be detected by the OPU 20 and used to verify the data marks 60directly after being written by the main beam/spot 40/50 to provide DRAWoperation. While the reflected beam associated with one of the satellitebeams 44, 48 (depending on the direction of travel of the tape 16)contains information associated with the data marks 60 on the tapemedium 16, the reflected beam is heavily contaminated by the modulationof the center beam 40 and other noise sources and generally exhibits avery low signal to noise ratio (SNR).

Some of the DRAW systems and algorithms contemplated use ademodulation/division method to verify written data during the writeoperation in real-time. For example during the write operation, thewritten data is decoded (read) from a reflected laser light signal by ahigh frequency demodulation circuit (divider circuit). Then, the signalquality of the decoded written data can be calculated by aBit-Error-Rate (BER) detector in order to verify the written data. As aresult, the time between data writing and data decoding in this exampleis less than 1 msec.

As mentioned above, data written by the main spot 50 could be read backby one of the satellite spots 54, 58 after a few micro seconds. Thesatellite spots 54, 58, however, only have a fraction of the lightintensity of the main spot 50. Thus, the light intensity signal detectedby either one of the satellite spots 54, 58 is modulated (distorted)with high-frequency laser pulses used for writing. In order to betterdecode the written data, the satellite spot reflected laser lightintensity signal (referred to as the RF signal) can be demodulated fromthe main spot writing laser pulse signal (referred to as the FM signalas it can be measured by a laser light front monitor detector) using,for example, a DRAW demodulation circuit in order to reverse themodulation caused by the writing pulsation of the laser diode. Also,frequency responses of the RF signal and FM signal can be matched byapplying a matched filter before the demodulation. A filter andhigh-frequency demodulator, therefore, can be designed for decoding andverifying written data during the write operation. Thus, thedemodulation and verification of written data can be in real-time.

FIG. 3A shows that the RF signal detected by a corresponding RF chip(e.g., a photodetector chip, PDIC, placed at the end of the reflectedoptical light path of the OPU 20) not only contains data associated withthe written mark being read but also the writing strategy waveformembodied by the main spot 50 at the time the written mark was beingread. That is, the RF signal is subject to noise introduced by thewriting strategy waveform. FIG. 3B shows that the FM signal detected bya corresponding FM chip (e.g., a front monitor chip, FMIC, placed at thelaser light output path of the OPU 20) from the scatter associated withthe center beam 40 is essentially the writing strategy waveform. FIG. 3Cshows that the division of the RF signal by the FM signal via a DRAWcircuit yields the written mark.

A voltage of the RF signal, V_(RF), can be represented asV _(RF) =k _(RF) ×ϕ×R  (1)where k_(RF) is a constant associated with the RF chip, φ is the writingstrategy modulated light intensity, and R is the changed reflectivity ofthe medium indicative of a written mark. And, a voltage of the FMsignal, V_(FM), can be represented asV _(FM) =k _(FM)×ϕ  (2)where k_(FM) is a constant associated with the FM chip. Dividing (1) by(2) yields k×R, where k is k_(RF)/k_(FM). Because k_(RF) and k_(FM) areknown, R can be obtained free of influence from V_(FM).

FIG. 4 shows the OPU 20 and an FMIC chip 23 arranged to receive laserlight from a laser diode 21. That is, the FM signal from the FMIC chip23 represents the direct light output of the laser diode 21 without anymodification by the OPU 20 or media 16. And, a PDIC chip 25 is arrangedto receive light reflected from the media 16 and through the OPU 20.

FIG. 5 shows the at least one controller and associated electronics 26implementing an analog DRAW demodulation circuit 62 to perform thesignal division described above. In this example, the circuit 62includes an RF signal input stage 64, a direct current (DC) bias 66, anall-pass delay filter 68, and a low pass filter 70. The circuit 62 alsoincludes an FM signal input stage 72, a low pass filter 74, a multiplier76, an op-amp 78, an inverter 80, and a DRAW division output 82. Thesignal process flow associated with the RF signal is the input stage 64to the DC bias 66, the DC bias 66 to the all-pass delay filter 68, theall-pass delay filter 68 to the low pass filter 70, and the low passfilter 70 to the op-amp 78. The sequence of these elements, however, maybe rearranged as necessary. The low pass filter 70, for example, maycome before the DC bias 66, etc. The signal process flow associated withthe FM signal is the input stage 72 to the low pass filter 74, the lowpass filter 74 to the multiplier 76, and the multiplier 76 to the op-amp78. The final leg of the signal process flow is the op-amp 78 to themultiplier 76 and to the inverter 80, and the inverter 80 to the DRAWdivision output 82.

FIG. 5 shows but one example of a demodulator or divider arrangementthat includes a multiplier, op-amp, and inverter. Any suitable sucharrangement, however, may be used. And although the elements of the DRAWdemodulation circuit 62 are shown to be implemented in analog form, theyof course may be implemented in digital form. In embodiments thatimplement at least the RF path in digital form, the all-pass delayfilter 68 may take the form of a Farrow structure phase delayinterpolator, which may allow for finer delay adjustment relative toother delay operations.

To better align the RF and FM signals for division, the DC bias 66applies a DC bias to the RF signal. In the example of FIG. 5, the biasis +1.3 volts. This value, however, may change depending on designconsiderations, medium configuration, etc. Also due to the differingfrequency responses of the RF and FM chips associated with the OPU 20,the all-pass delay filter 68 applies a delay to the DC biased RF signalfor synchronization purposes. In other embodiments, the all-pass delayfilter 68 may be in the FM signal path. The frequency associated withthe writing strategy can be on the order of 165 megahertz. This value,however, may change with tape speed, writing speed, etc. As such, thelow pass filters 70, 74 filter out frequency content associated with theRF and FM signals respectively, in this example, greater than 50megahertz for better performance in the demodulation stage. This valuemay also change with tape speed, writing speed, writing strategypattern, etc. A calibration procedure may be performed to select theappropriate bias and delay values prior to operating at run time.

As mentioned above, the modulation between high-frequency laser pulsesand previously written data results in a specific frequency spectrum ofRF signal that contains both low and high frequency components. The lowfrequency components are mainly from previously written data and lowfrequency components of the writing strategy pulses. The high frequencycomponents are mainly from high frequency components of the writingsstrategy pulses. Typically a writing strategy pattern is designed withconsideration of written data read-back signal quality, but not withdemodulated DRAW signal quality. Thus, conventional writing strategiesemployed in the DRAW techniques described above may experiencedifficulties in demodulating a DRAW signal from writing strategy pulsesdue to distortions and interference from the writing strategy pulses.These distortions are due to the overlap of spectrum between the writingstrategy and DRAW signal.

Here, certain writing strategies are optimized for both the read-backduring read and DRAW signals during write. By examining the frequencyspectrum of writing strategy pulses and the mechanism of modulationbetween the writing strategy and previously written marks during thewrite, new writing strategies with an optimized number of writingstrategy pulses and corresponding pulse widths better separate thewriting strategy and DRAW signal spectrums. As such, demodulation ofcertain DRAW signals is able to achieve the same signal quality as theread-back signal, without unnecessary distortions caused by modulationwith writing strategy pulses.

Differences between conventional writing strategies and some of theproposed writing strategies are as follows: (1) Precise writing strategycontrol could achieve timing resolution within 0.2 ns: The spectrum ofwriting strategy can be engineered with fine control over the number ofpulses as well as the associated pulse widths; (2) The writing strategyspectrum can be engineered to not overlap with the written mark'sspectrum located at the low-frequency region in the modulated DRAW'sspectrum: The proposed writing strategy pattern makes it possible todemodulate the DRAW signal without distortion caused by writing strategypulses; (3) The signal qualities of both the read-back signal and DRAWsignals can be optimized based on the fine tuning of the timing andlocation of the writing strategy pulses.

By way of example, when using a conventional writing strategy to writeand measure the dSNR of a DRAW signal during the write operation, theDRAW signal experiences about 5 dB SNR loss compared to the read-backsignal. This loss is due to the distortion from spectrum interference ofthe writing strategy pulses. When using an optimized writing strategywith an engineered frequency spectrum for the RF signal however, thedistortion loss is eliminated. And, the demodulated DRAW signal has thesame signal quality as the read-back signal. (The 5 dB SNR loss isrecovered). Hence, the writing strategies contemplated herein do notaffect the accuracy of DRAW verifications in real-time. Also because thenewly designed writing strategies consider both the demodulation processof DRAW and the writing process, the signal quality of the read-backsignal would also be optimized for optical tape media.

FIG. 6 shows an example 5T space and 4T mark laser pulse writingstrategy 84 with corresponding writing power, erase power, and coolingpower levels 86, 88, and 90. Pulses 92 are defined by the laser powerlevel in that the erase power level 88 precedes the pulses 92 for aleading time, the pulses 92 achieve the writing power level 86, and thepulses 92 are separated by the cooling power level 90. A trailing time94 also follows the pulses 92 at a power level less than the coolingpower level 90 prior to the next mark. The writing pulse timeresolution, in this example, is 1/32 of a clock cycle. Hence, 32/32 of aclock cycle represents 1 bit data for write, 64/32 of a clock cyclerepresents 2 bits, etc. Clock cycle timing indicators 96 indicate thetime associated with the leading time (12/32), the time associated withthe pulses 92 (10/32, 16/32, 16/32, 10/32 respectively), the timeassociated with the cooling power level 90 between the pulses 92 (10/32,16/32, 15/32 respectively), and the time associated with the trailingtime 94 (11/32). Initial selection and tuning of these times isdiscussed in further detail below.

As illustrated, 4 writing laser pulses are used to write a 4T mark byusing an nT pulse writing strategy. Unlike previous writing strategiesthat use n−1 pulses to write an nT mark, FIG. 6 is but one example ofthe proposed strategy to use at least n pulses to write an nT mark(e.g., n pulses, n+1 pulses, . . . , 2n−1 pulses, etc.) This proposedstrategy springs from the recognition that the Nyquist-Shannon samplingtheorem appears to be implicated when performing divider based DRAWverification. Within this context, each of the n pulses can be thoughtof as embedding a “sampling point” into the RF signal that representsthe writing strategy signal and causes distortion of the RF signal.Writing strategies that use the conventional n−1 pulses to write an nTmark will likely not have sufficient “sampling points” embedded in theRF signal to accurately reflect the shape of the DRAW signal in as faras the Nyquist criterion is concerned. When such an RF signal isdemodulated by a corresponding FM signal (essentially the writingstrategy pulses), the SNR will be negatively impacted because of thedifferences between the effectively incomplete DRAW signal embedded inthe RF signal and the corresponding FM signal.

Assume that a 2T mark with 2T space is to be conventionally writtenusing a 66 MHz writing clock. The frequency of the read-back 2T signalwill be 66 MHz/(2T*2) or 16.5 MHz. According to the Nyquist criterion,such a signal would need to be sampled at 33 MHz (2*16.5 MHz). If onlyone pulse is used, its frequency would be 33 MHz provided that the pulsewidth is equally divided into a 16/32-high and a 16/32-low. That is, 1bit would be sampled in less than 1 clock cycle. Practically speakinghowever, the pulse width of the writing strategy would be longer than16/32 for both the high pulse and low pulse in order to achieve theoptimal writing process for the best signal quality of the read-backsignal. Therefore, the equivalent sampling frequency of the DRAW signalwould be less than 33 MHz-resulting in signal distortion. In order tosatisfy the Nyquist criterion in the case of 2T DRAW, a writing strategyusing n pulses (or more than n pulses) for an nT mark is proposed toimprove the dSNR of the DRAW without signal distortion when compared tothe read-back signal.

To illustrate the effects of the proposed writing strategies, FIG. 7shows that by using the nT pulse writing strategy the DRAW recover areaand the modulation area associated with the writing strategy pulses donot overlap in the spectrum plot of the RF signal during write.Therefore, one is able to separate the DRAW recover area from thedistortion area and modulation area by low-pass filtering the signal.And, the DRAW signal could be fully recovered after the demodulationprocess. In contrast, the distortion area would expand to the DRAWrecover area when the n−1 pulses writing strategy is employed. Since itis noted that the 2T frequency locates at the right edge of the DRAWrecover area in the spectrum plot of the RF signal during write, thedistortion spectrum area would easily overlap with the 2T frequency, andtherefore introduce the distortion to the DRAW. Because it is verycommon for the equalizer in the following read channel to boost the 2Tfrequency due to the small amplitude of the 2T mark, the distortionwould also be boosted due to the spectrum overlap and cause more SNRloss for DRAW-exacerbating the distortion.

FIG. 8 shows the (phase shifted) similarity between a read signal(bottom waveform) generated during a read operation and, for the samedata, a DRAW division output signal (top waveform) generated by the DRAWdivision output 82 (FIG. 5) during a write operation that uses the nTpulse writing strategy.

FIG. 9 shows that the dSNR associated with DRAW verification can exceed24 dB using the proposed writing strategy. This is significantly higherthan that achieved by using the conventional writing strategy. Even atthe conventional writing strategy's maximum dSNR, the proposed writingstrategy yields an additional 5 dB.

As mentioned above with reference to FIG. 6, the initial selection andtuning of the clock cycle timing indicators 96 can be performed toachieve desired results. Generally speaking, the pulse widths are mediadependent and can be determined via testing with design of experiments(DOE). For an nT mark to be written using n pulses (nT pulse WS), a ruleof thumb starting point for writing power pulse width is 16/32 and forcooling power level width is 16/32. In the example of FIG. 6, the firstand last pulses 92 have been shortened based on read-back signalfeedback using known signal quality measurement techniques. For exampleby comparing a captured read-back signal with an expected wave form, oneis able to determine whether the writing process associated with thepulse widths is optimal for both read-back and DRAW. By looking atimprovement of dSNR of the read-back and DRAW related to adjustment ofthe writing strategy pulses and the DC offset of a 2T read-backwaveform, one is able to determine if the pulse width of the first andlast pulses should be increased or decreased. If the DC offset of the 2Tread-back waveform is biased negative compared to the average signallevel, this indicates too much energy being deposited on the media.Thus, the first and last pulse widths may be incrementally decreased andthen reexamined for performance. If the DC offset is biased positive,this indicates not enough energy being deposited on the media. Thus, thefirst and last pulse widths may be incrementally increased and thenreexamined for dSNR performance as well as the DC offset of the 2Twaveform in the read-back signal.

If an amplitude of the read-back signal corresponding to the erase powerlevel portion of the writing strategy is greater than the amplitude ofthe read-back signal corresponding to the pulsed portion of the writingstrategy for all nT read-back waveforms (not just 2T), this unbalancedwaveform indicates that not enough energy is being deposited during thewrite. Thus, the average writing power level may be increased to shiftall levels higher. If the opposite conditions are present, then too muchenergy is being deposited. Thus, the average writing power level may bedecreased to shift all levels of the writing strategy pulses lower.(Ideally, the amplitudes should be the same.)

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the disclosure andclaims. As previously described, the features of various embodiments maybe combined to form further embodiments that may not be explicitlydescribed or illustrated. While various embodiments may have beendescribed as providing advantages or being preferred over otherembodiments or prior art implementations with respect to one or moredesired characteristics, those of ordinary skill in the art recognizethat one or more features or characteristics may be compromised toachieve desired overall system attributes, which depend on the specificapplication and implementation. These attributes include, but are notlimited to cost, strength, durability, life cycle cost, marketability,appearance, packaging, size, serviceability, weight, manufacturability,ease of assembly, etc. As such, embodiments described as less desirablethan other embodiments or prior art implementations with respect to oneor more characteristics are not outside the scope of the disclosure andmay be desirable for particular applications.

What is claimed is:
 1. An optical storage system for performing directread after write on an optical medium, the optical storage systemcomprising: an optical head configured to split a light beam into a mainbeam and at least one side beam; and a controller configured to:modulate the main beam according to a writing strategy waveform thatdefines at least n writing pulses rising to a writing power level towrite an nT mark to an optical medium, process a first signal resultingfrom the at least one side beam being reflected from the optical medium,including altering a direct current bias of the first signal, process asecond signal indicative of the writing strategy waveform to removenoise from the first signal caused by the main beam, and generate outputindicative of the data directly after writing by dividing the firstsignal by the second signal.
 2. The optical storage system of claim 1,wherein the writing strategy waveform defines at least 2n−1 writingpulses rising to a writing power level to write an nT mark to theoptical medium.
 3. The optical storage system of claim 1, wherein thewriting strategy waveform further comprises the laser power level beingset to a level lower than a cooling power level subsequent to a finalpulse.
 4. The optical storage system of claim 1, wherein the writingstrategy waveform is such that a frequency of the at least n writingpulses is at least twice a frequency of a first signal.
 5. An opticalstorage system for performing direct read after write on an opticalmedium, the optical storage system comprising: an optical headconfigured to split a light beam into a main beam and at least one sidebeam; and a controller configured to: modulate the main beam accordingto a writing strategy waveform that defines at least n writing pulsesrising to a writing power level to write an nT mark to an opticalmedium; process a first signal resulting from the at least one side beambeing reflected from the optical medium; process a second signalindicative of the writing strategy waveform to remove noise from thefirst signal caused by the main beam; filter the first signal and thesecond signal to remove frequency content therefrom greater than athreshold to obtain, respectively, a first filtered signal and a secondfiltered signal; generate output indicative of the data directly afterwriting by dividing the first filtered signal by the second filteredsignal.
 6. The optical storage system of claim 1, wherein the opticalmedium is an optical tape.
 7. An optical storage system for performingdirect read after write on an optical medium, the optical storage systemcomprising: an optical head configured to split a light beam into a mainbeam and at least one side beam; and a controller configured to:modulate the main beam according to a writing strategy waveform thatdefines at least n writing pulses rising to a writing power level towrite an nT mark to an optical medium, wherein the writing strategywaveform is such that widths of at least some of the writing pulses aredifferent and wherein the writing strategy waveform is such that a widthof a final writing pulse is shorter than a width of an immediately priorwriting pulse; process a first signal resulting from the at least oneside beam being reflected from the optical medium, including altering adirect current bias of the first signal, process a second signalindicative of the writing strategy waveform to remove noise from thefirst signal caused by the main beam, and generate output indicative ofthe data directly after writing by dividing the first signal by thesecond signal.
 8. An optical storage system for performing direct readafter write on an optical medium, the optical storage system comprising:an optical head configured to split a light beam into a main beam and atleast one side beam; and a controller configured to: modulate the mainbeam according to a writing strategy waveform that defines at least nwriting pulses rising to a writing power level to write an nT mark to anoptical medium, wherein the writing strategy waveform is such thatwidths of at least some of the writing pulses are different and whereinthe writing strategy waveform is such that a width of a final writingpulse is shorter than a width of an immediately prior writing pulse;process a first signal resulting from the at least one side beam beingreflected from the optical medium; process a second signal indicative ofthe writing strategy waveform to remove noise from the first signalcaused by the main beam, filter the first signal and the second signalto remove frequency content therefrom greater than a threshold toobtain, respectively, a first filtered signal and a second filteredsignal; generate output indicative of the data directly after writing bydividing the first filtered signal by the second filtered signal.
 9. Theoptical storage system of claim 7, wherein the writing strategy waveformfurther comprises the laser power level being set to a level lower thanthe cooling power level subsequent to a final pulse.
 10. The opticalstorage system of claim 7, wherein the optical medium is an opticaltape.
 11. The optical storage system of claim 7, wherein the writingstrategy waveform is such that a frequency of the at least n writingpulses is at least twice a frequency of the first signal.
 12. Theoptical storage system of claim 7, wherein the writing strategy waveformdefines 2n−1 writing pulses for every n bits of data to be written to anoptical medium.
 13. An optical storage system for performing direct readafter write on an optical medium, the optical storage system comprising:an optical head configured to split a light beam into a main beam and atleast one side beam; and a controller configured to: modulate the mainbeam according to a writing strategy waveform that defines at least nwriting pulses rising to a writing power level to write an nT mark to anoptical medium, wherein the writing strategy waveform is such thatwidths of at least some of the pulses are different and wherein thewriting strategy waveform is such that a width of an initial writingpulse is shorter than a width of an immediately subsequent writingpulse; process a first signal resulting from the at least one side beambeing reflected from the optical medium, including altering a directcurrent bias of the first signal; process a second signal indicative ofthe writing strategy waveform to remove noise from the first signalcaused by the main beam, and generate output indicative of the datadirectly after writing by dividing the first signal by the secondsignal.
 14. An optical storage system for performing direct read afterwrite on an optical medium, the optical storage system comprising: anoptical head configured to split a light beam into a main beam and atleast one side beam; and a controller configured to: modulate the mainbeam according to a writing strategy waveform that defines at least nwriting pulses rising to a writing power level to write an nT mark to anoptical medium, wherein the writing strategy waveform is such thatwidths of at least some of the pulses are different and wherein thewriting strategy waveform is such that a width of an initial writingpulse is shorter than a width of an immediately subsequent writingpulse; process a first signal resulting from the at least one side beambeing reflected from the optical medium; process a second signalindicative of the writing strategy waveform to remove noise from thefirst signal caused by the main beam, filter the first signal and thesecond signal to remove frequency content therefrom greater than athreshold to obtain, respectively, a first filtered signal and a secondfiltered signal; generate output indicative of the data directly afterwriting by dividing the first filtered signal by the second filteredsignal.
 15. The optical storage system of claim 13, wherein the writingstrategy waveform further comprises the laser power level being set to alevel lower than the cooling power level subsequent to a final pulse.16. The optical storage system of claim 13, wherein the writing strategywaveform is such that a frequency of the pulses is at least twice afrequency of the first signal.
 17. The optical storage system of claim13, wherein the optical medium is an optical tape.